blog: DNA storage – to freeze or not to freeze?

DNA storage

About 5-6 million DNA samples (nearly 10% of the UK’s population) are currently being stored on the UK Police National DNA database (NDNAD). It is the largest DNA database in the world, as a proportion of the population.

In addition to these national databases, there are numerous biobanks and smaller research and clinical groups that store DNA. With such large numbers of samples being stored around the world, it is surprising to find many different storage practices.

Although freezing DNA at either -20oC or -80oC is widely accepted, there has been a drive, mainly in the area of forensics, to validate storage of DNA at room temperature (20 – 25oC). This would enable DNA to be stabilised on site before transporting to a forensic laboratory. Storing DNA at room temperature would also reduce the costs associated with long-term freezer storage. Most of the published studies in this field utilise a novel medium or minicapsule to protect the DNA from degradation at room temperature [1-5].

It is also possible to store DNA at 4oC. This is common practice for short-term storage but how stable is DNA over a longer period of time? There is very little scientific literature on this subject, but one study has demonstrated that DNA stored in a Tris-based buffer with an added chelator, rather than in water, is stable at 4oC for at least 16 years [6]. Storing samples at 4oC in an automated storage system avoids repetitive freeze thaw cycles, maintains sample integrity, reduces the cost and is more reliable than a subzero storage system.

Before deciding on what temperature to store your DNA at, it is important to understand how the DNA is going to be used.

how many times will the sample be used? freeze thawing the sample may have a greater detrimental effect on the DNA than storing as a liquid at 4oC or RT

what is it going to be used for? some DNA assays may be more robust than others

how long will it be stored for? requirements for long term storage may differ from that of short term

will it need to be transported? DNA may be collected offsite and/or be shipped to various locations

The relationship between biological activity and temperature

Temperature has a strong influence on protein dynamics: lower the temperature, lower the activity and greater the stability of the sample. Freezing changes the molecular entity of a sample; in the case of water changing from liquid to solid during the freezing process there will be a reduction in molecular mobility and in the case of enzyme activity, a reduction in reaction/activity rates will be experienced (see previous blog on the physics of freezing).

DNA degrades over time and just how long it lasts depends on how well it is preserved. To understand how freezing affects DNA degradation it is important to understand what actually causes DNA to degrade:

Nucleic acids are sensitive to depurination, depyrimidination, deamination and hydrolytic cleavage. All these processes are acid-catalysed processes and are affected by ionic strength of the solution. Eluting DNA in a Tris-based buffer, rather than water can inhibit these processes, however some downstream PCR reactions can be inhibited by Tris and therefore the DNA can only be eluted in water.

Nuclease contamination – this is more likely to occur with DNA eluted in water which contains nucleases. Nucleases can also be introduced into the DNA whilst working with the samples. As above, eluting DNA in a Tris-based buffer, rather than water can inhibit these processes.

Oxidation – contamination by transition metals can increase oxidation levels which produce free radicals and react with compounds causing DNA degradation. Degradation can be avoided by de-metalating all components involved in DNA storage using chelating agents.

Whatever temperature you decide to store your DNA samples at, TTP Labtech have an automated solution to meet your needs. Based on pneumatic technology, comPOUND (at room temperature, 4oC or -20oC) and arktic (-80oC) automated tube-based stores are designed to fit into any lab and can be linked to grow with your expanding collection. If you would like to find out more about TTP Labtech and their DNA storage solutions, please go to the website www.ttplabtech.com or contact sales@ttplabtech.com.